An uninterruptible power supply system for use in an electric power circuit includes a three-phase pulse width modulation rectifier portion for converting an alternating current from the electric power circuit to a direct current, a rechargeable battery unit capable of storing and discharging electric energy, a bidirectional battery charger and discharger circuit portion connected to the three-phase pulse width modulation rectifier portion and the rechargeable battery unit and a three-phase inverter portion connected to the bidirectional battery charger and discharger circuit portion for converting a direct current from the rechargeable battery unit to an alternating current of desired parameter values. The uninterruptible power supply system is configured to automatically switch between a grid mode of operation and a battery mode of operation, delivering a steady electric energy to a load circuit.